Flow control of exhaust gas through an internal combustion engine has been used in order to provide vehicle engine braking. Engine braking may include exhaust brakes, compression release type engine brakes, bleeder type engine brakes, and/or any combination thereof. The general principle underlying such brakes is the utilization of gas compression generated by the reciprocating pistons of an engine to retard the motion of the pistons and thereby help to brake the drive train and vehicle to which the engine is connected.
Exhaust brakes are known to be useful to help brake a vehicle. Exhaust brakes may generate increased exhaust gas back pressure in an exhaust system, including an exhaust manifold, by placing a restriction in the exhaust system downstream of the exhaust manifold. Such restriction may take the form of a turbocharger, an open and closeable butterfly gate or valve, or any other means of partially or fully blocking the exhaust system.
By increasing the pressure in the exhaust manifold, an exhaust brake also increases the residual cylinder pressure in the engine cylinders at the end of the exhaust stroke. Increased pressure in the cylinders, in turn, increases the resistance encountered by the pistons on their subsequent up-strokes (i.e., compression and exhaust strokes). Increased resistance for the pistons results in braking the vehicle drive train which may be connected to the pistons through a crank shaft.
In some known vehicle braking systems, exhaust brakes have been provided such that the restriction in the exhaust system is either fully in place or fully out of place. These exhaust brakes may produce levels of braking which are proportional to the speed of the engine (RPM) at the time of exhaust braking. The faster the engine speed, the greater the pressure of the gas in the exhaust manifold and cylinders. The higher pressure results in increased resistance to the up-stroke of the piston in the cylinder and therefore, increased braking.
Because the exhaust system and engine cannot withstand unlimited pressure levels, many systems include exhaust brake restrictions that are designed such that their operation at a rated maximum engine speed will not produce unacceptably high pressures in the exhaust system and/or engine that exceed a pressure limit. At engine speeds below the rated maximum engine speed, however, these exhaust brake restrictions may produce pressures that are lower than necessary. As a result, less than optimum braking may occur below the rated maximum engine speed.
In some known vehicle braking systems, exhaust brakes have been provided with a butterfly gate having a fixed-sized opening, or orifice, formed in the gate. When the gate is closed, the orifice provides an exhaust gas flowpath through the gate. The orifice may be sized such that at the rated maximum engine speed, the orifice permits a sufficient release of pressure from the upstream side of the butterfly gate that the exhaust pressure does not exceed the pressure limit for the engine. FIG. 1 of U.S. Pat. No. 7,350,502, which is hereby incorporated by reference, is a graph illustrating retarding power and back pressure versus engine speed (RPM) for an exhaust brake system having a valve and an orifice. The graph also illustrates an exhaust pressure limit and a targeted retarding power for a particular engine over a range of engine speeds.
In some known vehicle braking systems, exhaust brakes have been provided with variable restriction or bypass. Some variable restriction exhaust brake systems may include a spring loaded pressure-relief valve operable to admit flow of exhaust gases along a bypass flowpath only when a prescribed back pressure is reached. One such exhaust brake is disclosed in U.S. Pat. No. 4,750,459, which is incorporated herein by reference. In such exhaust brakes, when the prescribed back pressure is reached, the pressure overcomes the force of the bypass valve spring and opens the valve to relieve the pressure. When the valve opens, however, the flow of the gas through the valve may create a localized dynamic pressure drop near the valve. This pressure drop may cause the valve to close prematurely, or to rapidly close and then reopen. As a result the desired level of exhaust back pressure may not be easily maintained, and the desired level of braking may not be achieved. Accordingly, it may be advantageous to design an exhaust brake with a bypass that operates more in response to static exhaust pressure than to dynamic exhaust pressure.
Exhaust brakes are often used in diesel engine vehicles. Diesel exhaust tends to have high particulate matter content, referred to as soot. Over a prolonged period, diesel soot may adhere to and build up on the parts of an exhaust brake. As noted above, some exhaust brakes may include a bypass valve or mechanism with a bypass opening of a predetermined size designed to provide a desired level of exhaust backpressure at the highest rated engine speed. If soot builds up on such a selectively sized bypass opening, the opening may become sufficient blocked as to cause the exhaust backpressure to exceed that which is desired at the maximum rated engine speed. This can cause excessive and unintended exhaust braking, and potentially cause damage to the engine, exhaust system, and/or exhaust brake. Accordingly, it may be advantageous to design an exhaust brake in a manner that reduces the tendency of soot to build up on or in a bypass passage.
Embodiments of the present invention may provide apparatus and methods for controlling exhaust pressure in an internal combustion engine. Some embodiments of the present invention, but not necessarily all, may control exhaust gas back pressure independent of, or with reduced influence of, the effect of dynamic exhaust pressure on an exhaust brake bypass mechanism. Some embodiments of the present invention may also reduce the tendency of soot to build up on and/or block an exhaust brake bypass mechanism. These and other advantages of embodiments of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.